1. Field of the Invention
The present invention relates to an apparatus, a method, and a program used for identifying a modulation mode.
2. Description of the Related Art
As a type of multiplex modulation signals, there is an orthogonal frequency division multiplex (OFDM) signal. The communication system using the OFDM signal uses a plurality of sub-carriers whose center frequencies are different from each other, thereby providing a high frequency efficiency. In addition, it transmits transmission signals by multi-carrier signals so that it exhibits a strong resistance for interference. Therefore, the communication system using the OFDM signal has been actively applied to various kinds of communication systems such as radio LAN and the like (for example, Patent Literature 1). As has been selected for one of the radio LAN standard, it is a characteristic of the OFDM signal to have a structure in which a pilot symbol applied to synchronizing processing and a guard interval symbol for reducing the influence of multi-paths and for being applied to the synchronizing processing are added to an OFDM symbol to be transmitted (Non-Patent Literature 1).
FIG. 1A shows an example of the OFDM signal structure to which the present invention can be applied. As the fundamental structure of an OFDM signal frame, a pilot symbol for synchronizing processing is arranged at the front and, following the pilot symbol, the type of communication signal and a plurality of OFDM data symbols as transmission data are arranged. In FIG. 1A, Np is the pilot symbol length, Ng is the guard interval symbol length, Nd is the OFDM data symbol length, and M is the number of the OFDM data symbols.
As shown in FIG. 1B, for performing synchronizing processing and reducing the influence by multi-paths, a signal, which is a copy of a part of the OFDM signal to be transmitted, is added to the front of each OFDM data symbol as the guard interval symbol.
In conventional OFDM signal detection and demodulation processing, synchronizing and demodulation processing are performed on an assumption that the pilot symbol length Np, the guard interval symbol length Ng, the OFDM data symbol length Nd, and the number M of the OFDM data symbols per frame are known to a reception side in advance as information.
In the OFDM communication system in which the modulating parameters are uniquely changed by a transmission side in accordance with the communication quality, it is important to detect the modulating parameters based on a received signal in the demodulation processing performed on the reception side. With this, data communication for informing a change of parameters becomes unnecessary so that an increase in the data transmission capacity can be expected. Further, in a system for monitoring illegal radio waves, these parameters are generally unknown so that it is necessary to extract the parameters from the received signal.
As one of the conventional processing methods, there are two ways described in Non-Patent Literature 2 in regards to synchronizing demodulation method on condition that the above-described parameters are already known. One is a method in which a PN-system pilot symbol is added to the front of an OFDM signal block and the principle of CDMA matched filter is used. From a result of the correlation value output (see Non-Patent Literature 2, pp. 82-85), the method achieves frame synchronizing processing of the OFDM signal, and synchronizing and demodulation processing by detecting the phase error caused by radio wave propagation.
The other one of the conventional processing method is synchronizing demodulation processing using the guard interval symbol. In this method, in a circuit shown in FIG. 2 (see Non-Patent Literature 2, p. 57, FIG. 4.2, guard-interval-symbol-type synchronizing circuit), two data windows for correlation processing, which have time delay difference for the data length of the OFDM data symbol, are used for input signals, and synchronizing and demodulation processing are performed based on the correlation result. In FIG. 2, T is the OFDM block length without the guard interval symbol, and Tg is the guard interval symbol length.
[Patent Literature 1] Japanese Patent Unexamined Publication No. 2001-211137
[Non-Patent Literature 1] IEEE Standard 802.11a-1999, p. 12, 17.3.3 PLCP preamble (SYNC) FIG. 110-OFDM training structure
[Non-Patent Literature 2] “OFDM System Technology and MATLAB Simulation Guide” by Ochi Hiroshi, pp. 53-71, published by TRICEPS, Inc.,
However, it is necessary in the first processing method disclosed in the above-described Non-Patent Literature 2 to perform the correlation processing on an assumption that the reception side knows the length and the system of the pilot symbol which is added to the transmission signal. Further, the second processing method disclosed in Non-Patent Literature 2 achieves the effect of its action only when the data length of the OFDM symbol is already known.
As described above, the conventional OFDM modulation circuit is operable only when the modulating parameters of the OFDM signal are known, and it does not properly operate when there is input of a signal whose modulating parameters of the OFDM signal are unknown.